1. Field of the Invention
This invention relates to plasma etching of silicon nitride.
2. Discussion of Related Art
One selective nitride etching process for producing a sidewall process utilizes a Cl.sub.2 /O.sub.2 /Ar plasma etching chemistry at a selectivity of nitride over oxide of about 4-5:1. Overetching of such a silicon nitride layer can lead to total loss of an underlying thin SiO.sub.2 stopping layer and there is the potential of damage to the underlying Si substrate at the surface juxtaposed with the emitter opening in the surface of the Si substrate of a transistor being formed in the Si substrate.
U.S. Pat. No. 4,374,698 of Sanders et al, for "Method of Manufacturing a Semiconductor Device" etches silicon nitride and silicon oxide in a plasma with the silicon nitride layers being etched five times faster than the silicon oxide layers. The use of a gaseous compound containing a halogen other than a fluoride is described.
The gas composition preferably includes
______________________________________ 62.5% CF.sub.4 ______________________________________
A halogen compound other than a fluoride:
______________________________________ 7.5% (2-8% pref) CF.sub.3 Br or 1-4% CF.sub.2 Cl.sub.2 ______________________________________
An oxidation compound or molecule:
______________________________________ 30% NO (range 20-40%) or 3-10% O.sub.2 0% diluent gas listed ______________________________________
Preferred reactor conditions
______________________________________ pressure 100 Pascal temperature 125 deg. C. substrate temp. RF power 150 Watts rotational magnetic field none RF Frequency 13.56 MHz Gas Flow 100-300 sccm ______________________________________
U.S. Pat. No. 4,793,897 of Dunfield et al for "Selective Film Etch Process" uses ". . . a reactant gas mixture of fluorinated etching gas and oxygen for selectively etching a thin film of material such as silicon nitride with high selectivity for a silicon oxide underlayer . . ." without any magnetic field and without any other halides than SiF.sub.4.
The gas composition preferably includes
______________________________________ 10-400 sccm Total gas flow 0-100 sccm NF.sub.3 0-100 sccm SiF.sub.4 0-100 sccm O.sub.2 0-100 sccm He 10-150 sccm Chamber ______________________________________
Preferred reactor conditions
______________________________________ pressure 0.5-30 mTorr temperature 25 degrees C. RF power 100-1500 watts RF Frequency none listed ______________________________________
U.S. Pat. No. 4,844,773 of Loewenstein et al for "Process for Etching Silicon Nitride Film" with respect to FIG. 32 and the process unit 1300 describes a process adapted for a low pressure silicon nitride etch. It is stated that "HBr or CF.sub.3 Br provides a very potent passivating chemistry for fluorine-based etches." The Loewenstein patent suggests use of SiF.sub.4 with HBr to etch a thin film of tungsten. However, with respect to silicon nitride, the examples of etches are as follows: